This invention relates to a photographic support and a process for producing the same. More particularly, this invention relates to a water-resistant photographic support having a layer containing an electron radiation hardened resin and white pigment and a process for producing the same.
Recently, there are mainly used photographic supports obtained by coating at least one surface of a base sheet used as a support with a polyolefin resin kneaded with a white pigment. Such a polyolefin resin layer is formed on the base sheet for mainly preventing the base sheet from penetration of treating chemicals thereinto at the time of developing. Further, by forming the polyolefin resin layer, the developing time can be shortened and yellowing, with the lapse of time, can be prevented because of no residual treating chemicals in the base sheet. The polyolefin resin layer is usually formed by using a melt extruder. Further, in the polyolefin resin layer which becomes a front side of a photographic paper, an inorganic white pigment such as titanium dioxide is kneaded in order to improve hiding power and whiteness of the photographic support and to improve resolving power of photographic paper. However, since the polyolefin resin is remarkably highly viscous even when melted by heat, it is not easy to disperse the inorganic pigment such as titanium dioxide and further, the content of the inorganic pigments is limited. In order to obtain good hiding power and whiteness, the thickness of the polyolefin resin layer should be increased. Further, since the melt extrusion coating of the polyolefin resin is carried out at the pyrolysis temperature of the polyolefin resin or higher, yellowing due to pyrolysis of the resin and pin holes are produced in the resin layer. Further, with an increase of the extrusion speed, since the molten polyolefin resin is peeled off before sufficiently cooled and solidified by a cooling roll, releasing properties of the coating resin from the cooling roll becomes poor to produce so-called "blocking" and optical unevenness of gloss surface usually called "transverse stripe unevenness". Therefore, there is a problem in that quality of a printing paper which is a final product is undesirably lowered. In order to prevent the generation of unevenness due to "blocking", it is generally known to add a lubricant to the polyolefin resin. But, according to this method, an amount of smoke from the molten polyolefin resin surface due to pyrolysis of the lubricant increases at the time of melt extrusion, which results in causing stain on the cooling roll surface. As a result, quality of the resin coated surface is lowered to bring about secondary lowering in quality such as causing a so-called "satin surface".
In the production of a resin-coated photographic support by the melt extrusion method, it is difficult to thin the resin coating layer while maintaining a proper hiding power or to increase the coating speed without lowering the quality. Thus, there is proposed a water-resistant photographic support obtained by coating an electron radiation hardened resin kneaded with a white pigment on a base sheet, followed by irradiation with election beams to form a coating layer (e.g. U.S. Pat. No. 4,384,040). The resulting photographic support can have a thin resin layer without lowering the covering power. Further, since the hardened resin layer is obtained by polymerization and crosslinking with irradiation of election beams at room temperature, the defects caused by the melt extrusion method can be removed.
But there arises another fatal problem in that when a photographic paper is made from said photographic support, yellowing progresses with the lapse of time due to influences of electron beam irradiation such as decomposition of cellulose in the base sheet and adsorption of treating chemicals on the coating resin during the developing step. Further, there is another problem in the production process in that the irradiation of election beams should be carried out in an inert gas in order to prevent poor hardening caused by oxygen. But even if the election radiation hardening is carried out in an inert gas, the resulting hardened resin layer is insufficient in adhesive strength with a photographic emulsion layer to be formed, and thus should be subjected to a surface activating treatment such as a corona treatment, a flame treatment, etc. Further, even if such a surface activating treatment is conducted, sufficient adhesive strength cannot be obtained due to unevenness of surface activity.
A further disadvantage of this photographic support is that when a photographic paper obtained from this photographic support is subjected to development, adhesive strength is undesirably lowered to cause peeling in the worst case.
A still further disadvantage of this photographic support is that the control of coating step using a coater is difficult, since the viscosity of an electron radiation hardened resin is remarkably increased by dispersion of a white pigment when dispersed in high concentration, and rib-like unevenness is caused after coating.
A still another disadvantage of this photographic support is that when a white pigment is dispersed in high content in an electron radiation hardened resin, not only are the effects of the fluorescent agent remarkably decreased due to the covering of the almost fluorescent agent with the white pigment, but also fogging of photographic emulsion layer is produced by decomposed product of the fluorescent agent in the case of forming a photosensitive emulsion layer on the hardened resin layer caused by electron radiation depending upon the kind of fluorescent agent used. Further, some fluorescent agents may remarkably lower coating properties of an electron radiation hardened composition containing a white pigment fail to obtain a smooth surface, even if contained in a trace amount. In addition some fluorescent agents may lower hardening properties of an electron radiation hardened composition such that an excess amount of electron beams irradiation is required.